(1) Field of the Invention
This invention concerns televison cameras, and which are relates in particular to such cameras using solidstate image sensors and required to provide a good image of a scene under all normal night and day ambient light condition.
(2) Description of the Prior Art
Television cameras are widely used for numerous purposes, and in many instances twenty-four-hours-a-day viewing is required. However, over such a period the ambient light level can vary from 10.sup.-4 lux (the darkness of a cloudy night) to 10.sup.+4 lux (bright sunlight), and this creates a problem because there is presently no single image sensor which can cope with such a wide range of light levels. It is therefore common practice either to employ two different types of camera and sensor (one for daytime, and one for night time), or to use a simple night-time camera and sensor but with optional filters for daytime use, or to provide a source of illumination for night time operation. Each solution is expensive, and the latter obviously complicates the situation when the camera is required to be hidden from view.
The problems associated with size can be mitigatd by using cameras that employ solid-state image sensors instead of the more conventional vacuum-type camera tubes. The former have much smaller size, weight and power consumption, and have the added advantage that the camera head (incorporating a lens and the image sensor) is a very small unit that can be separated from the main camera body (containing most of the necessary electronic circuitry) but joined thereto by a small multi-core cable both providing the input drive supplies for the sensor and carrying the video output signals to the camera amplifier and video processing electronics in the main control unit. This type of camera is commonly referred to as a "remote-head" unit. An added advantage of the solidstate sensors is that their lifetime is virtually infinite, so that the frequency of the necessary camera maintenance is significantly reduced as compared with that for vacuum tube cameras.
Cameras using solid-state image sensors can cope satisfactorily with light levels in the range 1 lux to 10.sup.+4 lux (the normal daylight range) when used in conjunction with an automatic iris lens to control the amount of light incident on the sensor, but they do not have sufficient sensitivity to operate under night time conditions. In this latter case it is common practice to provide amplification of the illumination incident on the image sensor by means of an image intensifier, which can be coupled directly to the image sensor either using a fibre optic bundle or by means of lenses. A camera employing such a combination of intensifier and solid-state image sensor will provide a reasonable picture in ambient lighting conditions down to overcast starlight levels (10.sup.-4 lux), but naturally cannot be employed in its basic form in daylight because the intensifier then becomes overloaded, and possibly permanently damaged. Attempts have been made to deal with this problem--to extend the range of an intensifier-using low light level camera to higher light levels--by means of an arrangement of neutral density optical filters which are automatically moved into the light path in front of the intensifier as the light level increases. This works, but does not provide the full answer, for--unlike solid-state image sensors (which, as mentioned previously, have a virtually infinite life)--the life of an image intensifier is limited to a few thousand hours of operation, and is further reduced as the signal current flowing through it is increased at the higher light levels. For short duration applications such a mode of operation is feasible, but for continuous "round-the-clock" operation it is uneconomic to employ an expensive intensifier-sensor combination for daytime viewing. Moreover, the picture quality (usually defined by the signal-to-noise ratio) obtained from an intensifier-using a low light level camera is not as high as that from an unintensified camera. For these reasons it is still common practice to employ different types of camera for daytime and night time work, despite the drawbacks of such an arrangement. The present invention, however, puts forward a solution to the problem that in itself carries no, or substantially no, associated penalities. Specifically, the invention proposes a camera incorporating both types of sensor--that is, two solid-state sensors one of which is intensified and one of which is not--in the same camera head, and "switching" one or other into operation depending upon the ambient lighting conditions. This has not previously been practical with the larger vacuum-type camera tubes, or with the relatively bulky first generation intensifiers, but is now possible with the much smaller solid state sensors and the smaller "second generation" microchannel plate image intensifiers.